This invention relates to promoting nucleation of solute in a solution, and in particular, though not exclusively, to the separation of dissolved wax from hydrocarbon oil mixture boiling in the lubricating oil range. In this specification, "solute" means a liquid or solid dissolved in a solvent.
One area of particular interest as far as the invention is concerned is the separation of a solute from the solvent in which it is dissolved. The problems of wax in lubricating oil are very well known in the art. In the distillation of crude oil, a proportion of wax is present in cuts taken in the lubricating oil range. Some of the wax remains dissolved in the oil, whereas other fractions form a haze as the oil fraction ages at ambient temperatures. Wax in itself is in fact a good lubricant but under comparatively low temperatures such as engine cold start conditions, its presence causes the oil to be thick and viscous and as a result the engine may be hard to turn over at sufficient speed during starting. Haze manifests itself as a milky or cloudy appearance in the oil and is often caused by wax or by both wax and tiny water droplets being present in the lubricating oil. Typically a minimum of about 0.1% by volume of wax will cause some lubricating oils to look hazy. Therefore the existence of haze caused by the presence of wax crystals or particles detrimentally affects the performance of lubricating oils. It is of paramount practical importance to devise techniques for removing dissolved was from lubricating oil in a relatively inexpensive, simple and effective manner, which techniques are also capable of implementation on an industrial scale. A second area of interest as far as the invention is concerned is the extraction of a polar liquid dissolved in a non-polar liquid. An example is a residium which may be entrained and dissolved by a distillation process into a largely non-polar distillate fraction. In addition, the separation of other solutes than dissolved wax or residuum from solvent liquids generally is of practical importance and finds application throughout the petroleum and chemical industries.
It is well known that in some cases, for separating a solute from the solvent in which it is dissolved, the solution can be cooled sufficiently so that the solute is converted into a precipitate which can then be removed from the solvent in any suitable manner. In the case of a dissolved liquid, when the solution temperature is reduced below the dissolution point for that liquid, the dissolved liquid precipitates as a second liquid phase distinct from the solvent phase. For a dissolved solid, a precipitate is formed when the solution temperature falls below a critical temperature such as the melting temperature or the crystallization temperature of that solid in the solvent used. One way in which the precipitate can be separated from the solvent is to pass the precipitate-laden solvent through a filter medium or filter screen, but for this technique to be effective, the average particle size of the precipitate needs to be sufficient, such that the major proportion of the precipitate is restrained by the filter. In the case of dissolved wax in lubricating oil, in order to promote the precipitation of wax particles, an oil solvent is generally added to the wax-bearing lubricating oil (hydrocarbon oil mixture boiling in the lubricating oil range). "Oil solvent" as used throughout this specification refers to those solvents which when added to an oil mixture result in a lower viscosity for the solvent-oil mixture than for the oil mixture alone. This is beneficial for enhancing the settling or filtration processes used to separate the precipitated wax from the solvent oil mixture. Usually, the oil solvent will have the additional property of having a higher solubility for the oil mixture than for the wax at any given temperature, so that during chilling of the solvent-oil mixture to precipitate wax, the wax precipitation is enhanced. However, the resulting wax particles usually have a very small mean diameter (e.g. 0.1 to 100 microns) and special rotary drum filters have to be used, employing a filter cloth which extends around the drum periphery and through which the wax precipitate-bearing lube oil/solvent mixture is drawn under suction to form a wax deposit or cake on the filter cloth. Since the rate of filtration is directly related to the viscosity of the lube oil/oil solvent mixture, which is lower than that of the lube oil alone, the filtration rate is enhanced. Furthermore, it is known to use a vaporizable oil solvent liquid, such as liquified propane, and to allow the solvent liquid to vaporize from the lube oil so as to induce at least part of the overall chilling of the oil which brings about precipitation of the wax. However, the filterability of the resulting wax particles is inversely related to the rate of cooling so that in practice, the cooling, whether produced by the auto-refrigeration effect of vaporizing propane or other vaporizable oil solvent or in some other way such as by indirect heat exchange with a cooling medium, has to be effected at a controlled, slow rate, in order that the downstream rotary drum filter or other filter unit is able to separate the wax precipitate from the lube oil. This results in increasing treatment time in a lube oil dewaxing plant and added technical complexity.
It is also known that, in general, the mechanism of precipitation of a solute in a solvent is initiated by nucleation of the solute as the solubility for the solute is reduced. Nucleation is the formation of nuclei which themselves, when grown to a critical size, will act as sites at which agglomeration and crystallization occurs to convert the solute into crystals or particles forming the precipitate. Therefore, the onset of nucleation is critical to the precipitation of the solute in the solvent. It follows that a need exists for a method of promoting nucleation of a solute, which can lead to enhanced precipitation of the solute. Furthermore, a method for changing the nucleating mechanism, in particular the critical nucleation size, number density, nucleation rate, growth rate following nucleation and morphology of the precipitating species, can have important practical applications, not only in dissolved solid or liquid separation but also in crystal morphology.